I must be colourblind, most of those look the same on my phone.

Same. There were like three different colors at first and then the remainder looked mostly the same.

Also, I wonder how the results are affected by my screen and environment. I’m on an iPhone in a dark room, with brightness turned all the way down and I currently have TrueTone enabled and Night Shift enabled.

I was bluer than x percent of the median. Night Shift mode reduces blue light exposure. At daytime with Night Shift off, I would surely be seeing the boundary earlier, as there would be more blue light transmitted by my screen.

I may have to repeat the attempt multiple times on different screens and lighting conditions (both indoors annd outside) and see if the results vary wildly or not. I think they will.